Prior art techniques for forming nonwoven fabric from batts of random, loose thermoplastic fibers and the like have encountered one or more of the following drawbacks: Unacceptable fabric properties (especially strength in hand). Slow processing speeds. Equipment malfunction due to burnup of the anvil roller should the batt break or cease to be fed between the horn and roller, or should scrap metal inadvertently appear in the bat. Unacceptable aesthetic characteristics of the fabric, especially surface characteristics. Unacceptable failure rate of ultrasonic horns.
Problems associated with an unacceptably high failure rate of ultrasonic horns have also occurred in the formation of nonwoven fabric from batts having a weight greater than 3 ounces per square yard, and even in the fusing of fibers of a plurality of webs together.
According to the present invention, all the above drawbacks, except for horn failure, are solved utilizing a carrier of flexible sheet material capable of drawing and spreading out ultrasonic energy from the machine, and passing the carrier and lightweight batt together between the horn and anvil of the machine. The use of the carrier unexpectly results in increased fabric strength for lightweight fabrics (i.e. 3 ounces per square yard and less, depending on fiber denier), results in a fabric having smooth surfaces and good textile hand, and thus better aesthetic characteristics, allows a quantum increase in the speed of bonding, and positively prevents damage to the anvil roller even if the batts being fed to the ultrasonic machine should break or scrap metal should inadvertently become associated therewith.
The unacceptably high horn failure rate can also be remedied according to the present invention by constructing the horn face in a particular manner vis-a-vis the patterned roll anvil. When ultrasonic horns having horn faces with a width greater than twice the spacing between the raised surface portions of the roller anvil in a dimension perpendicular to the axis of rotation of the anvil roller are utilized, a high failure rate of horns results. Such horns have substantially even amplitude along the entire extent thereof (e.g. 3.6-4 mils). It has been found according to the present invention, however, that by providing a horn face having a dimension perpendicular to the axis of rotation of the anvil roller less than twice the spacing between raised portions in that dimension, problems of unacceptable of horn failure are eliminated, and a product having enhanced smoothness results. Such horns with relatively thin faces (e.g. 1/10 inch) have significant (e.g. greater than 1 mil) amplitude variation over the length thereof (e.g. 2.6 mils at the ends and 4 mils at the middle).
It is the primary object of the present invention to produce fabrics utilizing ultrasonic bonding with increased process efficiency, especially to produce lightweight nonwoven fabrics from lightweight fiber batts. This and other objects of the invention will become clear from an inspection the detailed description of the invention from the appended claims.